336 TlMEHRl. 
and other porous substances, on various salts, alkaloids 
and colouring matters in solution. It is usually stated 
that no strictly chemical union takes place between the 
organic matter of the soil and the substances it arrests, 
but the writer considers that the separation undergone 
by the ammonium and potassium salts, into the acid 
which passes away, and the base that is retained 
is sufficient to prove that a definite action does 
take place, and that the ammonia or potash is retained 
in the soil consequent on forming an insoluble compound 
with some organic acid. 
However, the exact way the ammonia becomes fixed in 
the soil is of little consequence, provided it only remained 
so until the nitrogen contained in it was taken up by the 
crop as required ; but, it unfortunately happens that 
ammonia so situated speedily begins to change, and be- 
comes oxidised into nitric acid, which then immediately 
unites with any bases it may meet with to form soluble 
salts or nitrates, that are readily washed out of the soil 
by the rainfall. 
That ammonia was capable of being converted into 
nitric acid by natural agencies has long been considered 
probable from the observations that have been made on 
the formation of artificial nitre (or nitrate of potassium) 
by the decomposition of animal matters in the so-called 
nitre heaps, and from the frequent occurrence of nitrates 
(nitre-rot) deposited on the brick walls of stables where 
ammoniacal emanations abound. In fact ammonia has 
long been regarded as the penultimate, and nitric acid 
the final product of the putrefaction or decay of nitro- 
genous animal and vegetable substances. But a more 
especial agricultural interest was awakened to the 
